The present application relates to an illumination optical device used to illuminate a spatial light modulator in an image display and to a virtual image display including the illumination optical device serving as an illumination optical system.
Some virtual image displays proposed in the past are designed to display to a viewer a virtual image formed by magnifying a two-dimensional image with a virtual image display optical system. For instance, Japanese Unexamined Patent Application Publication No. 2007-12530 proposes a virtual image display allowing a viewer to observe an image as a virtual image by repeatedly propagating image light obtained in a spatial light modulator while totally reflecting the image light within a light guide plate.
FIG. 16 shows an example of the virtual image display disclosed in the publication. This virtual image display 100, as shown in a schematic plan view of FIG. 16A and a schematic side view of FIG. 16B, includes an illumination optical system 101, a spatial light modulator 102 displaying images with illumination light, and a virtual image display optical system 103.
The illumination optical system 101 includes a light source 104, a tapered light pipe 105, a lens unit 106 with two linear Fresnel lenses 106A and 106B, a diffuser 107, and a polarizing beam splitter 108. In the illumination optical system 101, illumination light from the light source 104 is repeatedly undergone total reflection within the light pipe 105, emitted therefrom and enters the spatial light modulator 102 after passing through the two Fresnel lenses 106A, 106B, diffuser 107 and polarizing beam splitter 108. The light source 104 may be, for example, a light-emitting diode (LED). The spatial light modulator 102 may be a reflective spatial light modulator, more concretely, a reflective liquid crystal panel.
The linear Fresnel lenses 106A and 106B are arranged so that their optical powers are oriented perpendicular to each other and have different optical powers (i.e., focal lengths) from each other. The optical lens unit 106, composed of the linear Fresnel lenses 106A and 106B, varies its diffusibility according to the directions of the optical power. The diffuser 107 delivers higher diffusibility to light in the directions in which optical power is higher. The optical property of the Fresnel lenses 106A and 106B and the diffusibility of the diffuser 107 adjust the exit angle and the numerical aperture (NA) of the illumination light, and then the adjusted illumination light exits the diffuser 107 to enter the polarizing beam splitter 108.
The polarizing beam splitter 108 having received the illumination light reflects only an S-polarized component, for example, to illuminate the reflective spatial light modulator 102. The reflective spatial light modulator 102 modulates the S-polarized component light into image light according to an image. Then the image light is reflected off and passes through the polarizing beam splitter 108 to enter the virtual image display optical system 103.
The virtual image display optical system 103 includes a collimating optical system 110, a thin flat light guide plate 112, a first reflective volume holographic grating 113 and a second reflective volume holographic grating 114. The first reflective volume holographic grating 113 is placed on one end of an optical surface (back face) 116 of the light guide plate 112, the end being opposed to the collimating optical system 110. The second reflective volume holographic grating 114 is placed on the other end of an optical surface (back face) 116 of the light guide plate 112, the end being opposed to a viewer's pupil 117.
The image light is converted by the collimating optical system 110 into groups of parallel rays, which are in turn incident in the light guide plate 112 through one end of the optical surface 115, are diffracted and reflected by the first reflective volume holographic grating 113 and travel toward the other end while repeatedly undergoing total internal reflection within the light guide plate 112. The image light having reached the other end of the light guide plate 112 is diffracted and reflected by the second reflective volume holographic grating 114 and exits from the optical surface 115 to enter the viewer's pupil 117. Through this process, the image obtained at the spatial light modulator 102 in the virtual image display 100 can be viewed as a magnified virtual image formed by the virtual image display optical system 103.